Nov. 20As Bob Chant squeezed through the gap in the chain-link fence, it was clear how little respect the Passaic River has gotten.

Chant, a Rutgers University oceanographer, was in Newark on Friday to study the river and answer what may be one of New Jersey's costliest questions: how best to deal with the toxic muck that fouls it.

Contaminated for centuries by industry and sewage, the Passaic now holds a toxic stew of PCBs, dioxins and other chemicals in its riverbed. Environmentalists say the only way to save the river is to dredge it. But that could cost hundreds of millions of dollars  and some wonder if it might even do more harm than good.

Chant and his colleagues are looking for answers.

But first, the scientist had to navigate an industrial wasteland. Seeking the river, Chant came to the foot of Blanchard Street, a dead-end drive that leads to an abandoned factory at the Passaic's edge. He circled through a field of chest-high weeds, past a dead chicken and a child's sneaker, and ended up at a dilapidated, asphalt-covered pier that sagged into the water.

Nuzzled at the edge was a Rutgers research boat, piled high with sonar equipment, temperature sensors and computer equipment all intended to probe the depths of one of New Jersey's dirtiest stretches of water.

The Passaic, it turns out, still has mysteries for scientists. The information Chant's team is gathering  the shape of the river's currents, the salinity of its depths, how muck spreads and settles in the water  could help decide whether dredging is the solution or a colossal waste.

"It's a huge decision the state has to make," Chant says. "Should you dredge or not? You need good science to make that decision. That's why we're out here."

Estimates of the cost of dredging the Passaic range widely, from $80 million for removing a few contaminated hot spots, into the billions if a cleanup includes the entire lower river from Newark Bay to the Dundee Dam between Passaic and Garfield. Along the Hudson River in upstate New York, an effort to clean up PCB-tainted sediment is expected to cost $500 million.

As a result, government and industry are waiting for guidance from experts like Chant.

"We're all in favor of figuring out the best way to do this," said Michael Turner, a spokesman for the company that owns Diamond Alkali, the former pesticide factory blamed for much of the pollution in the river.

Chant's visit Friday was part of a yearlong, $180,000 study of the Passaic's physical profile. The team from Rutgers' Institute of Marine and Coastal Sciences deployed sensors at several spots along the river. The equipment will be there until the water freezes this winter, gathering data on temperatures, pressure and other factors that affect how dredged sediments might move in the river.

One target was the stretch outside the old Diamond Alkali site, which produced Agent Orange, the defoliant used by the U.S. military in the Vietnam War. The site is now on the federal Superfund list of the nation's most contaminated sites.

Floating beside the site, Chant and three colleagues deployed an acoustic Doppler current profiler  a tripod-shaped contraption that uses sound waves to measures the river's currents, as well as the amount of sediment drifting through the water.

The device gathers data once a second for 10 minutes, taking a half-hour off before resuming the cycle.

Two technicians attached the profiler to a canary-yellow buoy and an anchor, its heavy chain rumbling on the deck as the weight sank below. Next, the profiler went overboard, along with a salinity meter to record salt levels.

It's a tricky procedure. Drop the profiler too close to other devices and its sound waves will interfere with the rest of the experiments. Drop it too quickly, and you end up with a sensor facedown in the mud and useless.

The solution isn't fun. Researchers have to don wet suits and dive into the Passaic, a river often tainted by sewage and chemicals.

"At 22 feet down, it's just total black. You're just following along on the chains," said Chip Haldeman, one of the technicians. "The idea is, get this right the first time because I really don't want to do that again."

Chant will be back in the spring for more experiments. He'll also release a fluorescent green dye to track how particles move through the water over two days.

A key question is whether dredging would remove pollution or just stir up chemicals now buried in the riverbed, making matters worse.

Turner, the Diamond Alkali spokesman, said that's a major cause for concern. Environmentalists, however, say it's an excuse for inaction. They say the river has been studied for years and it's time to clean it up.

Chant said he doesn't know yet whether dredging is the answer.

"Right now, we're just trying to obtain the character of the river," he said. "Before we were out here, no one could really say at what flow you get stuff blasted down the river."

"This is our environment, the river," he continued, "and in order to make an intelligent decision about how the sediment is going to spread, you have to know how that environment works."